1. Field of the Invention
The present invention relates generally to communication systems, and more specifically to a traffic control system and method for efficient utilization of transmission link resource for transmission of real-time signals in a constant-rate mode and packets in a variable-rate mode.
2. Description of the Related Art
In communication systems, two transmission modes are available. One is a constant-rate transfer mode for real-time services in which latency is impermissible, such as voice and facsimile signals. The other is a variable-rate transfer mode where some latency is allowed, such as packets. In a mixed mode of transmissions, call requests are treated equally for both types of transmission so that if the transmission link is full to its capacity, request from a new cell is rejected. In other mixed modes of transmission, priority is given to the constant-rate mode so that when the usage rate of the transmission link exceeds some critical value, all call requests for variable-rate mode of transmission are rejected.
A similar traffic control system is disclosed in Japanese Laid-Open Patent Application 10-107769. According to this prior art, the total traffic volume of user terminals of a first service category is measured at periodic intervals and the measured volume is subtracted from the system""s maximum capacity. If there is a remaining capacity, a portion (transmission rate) or whole of it is assigned to user terminals of a second service category. However, in this prior art assignment system the transfer rate of a packet call is maintained constant for the duration of the call. Thus, the transmission link resource is not utilized to the fullest extent for a mixed mode of transmissions.
Because of the bursty nature of packet transmission, the present invention is based on the utilization of a transmission link by lowering the transfer rate of a packet to vacate a portion of the link resource for calls of constant-rate transfer mode when congestion occurs on the link.
It is therefore an object of the present invention to provide a communication system for efficient utilization of transmission link resource for variable-rate and constant-rate modes of transmission.
In a first aspect, the present invention provides a communication system comprising control circuitry for assigning a first transfer rate to a first user terminal requesting a variable-rate mode of transfer over a transmission link and subsequently assigning a second transfer rate to a second user terminal requesting a constant-rate mode of transfer over the transmission link if the second transfer rate is equal to or smaller than a remaining capacity of the transmission link. The control circuitry reassigns a third transfer rate lower than the first transfer rate to the first user terminal if the second transfer rate is greater than the remaining link capacity so that the remaining link capacity is increased. If the second transfer rate is equal to or smaller than the increased remaining link capacity, the control circuitry assigns the second transfer rate to the second user terminal.
Preferably, the communication system is a code division multiple access system and the control circuitry is arranged to assign a first spreading code to the first user terminal as an indication of the first transfer rate and reassign a second spreading code to the first user terminal as an indication of the third transfer rate, the first and second spreading codes being hierarchically organized as higher and lower layers, respectively, and the second spreading code being longer than the first spreading code.
In a second aspect, the present invention provides a method of controlling transfer rates. According to the method, a first transfer rate is assigned to a first user terminal requesting a variable-rate mode of transfer over a transmission link. A second transfer rate is subsequently assigned to a second user terminal requesting a constant-rate mode of transfer over the transmission link if the second transfer rate is equal to or smaller than a remaining capacity of the transmission link. If the second transfer rate is greater than the remaining link capacity, a third transfer rate lower than the first transfer rate is reassigned to the first user terminal to increase the remaining link capacity. The second transfer rate is assigned to the second user terminal if it is equal to or smaller than the increased remaining link capacity.
In a still further aspect, the present invention provides a method of controlling transfer rates comprising the steps of (a) assigning a first transfer rate to a first user terminal in response to a call request from the first user terminal requesting a variable-rate mode of transfer over a transmission link, (b) defining a relationship between identity of the first user terminal, an indication of the variable-rate mode of transfer and the assigned first transfer rate in a first entry of a memory, (c) determining from contents of the memory a remaining capacity of the transmission link in response to a call request from a second user terminal requesting a constant-rate mode of transfer over the transmission link and comparing the remaining link capacity with a second transfer rate requested by the second user terminal, (d) if the second transfer rate is equal to or smaller than the remaining link capacity, assigning the second transfer rate to the second user terminal, (e) defining a relationship between the identity of the second user terminal, an indication of the constant-rate mode of transfer and the assigned second transfer rate in a second entry of the memory, (f) if the second transfer rate is greater than the remaining link capacity, making a search through the memory for an entry containing a user terminal whose mode of transfer is variable rate and whose transfer rate can be reduced, (g) if the entry is detected by the step (f), reassigning a third transfer rate lower than the first transfer rate to the first user terminal and updating the first entry of the memory and returning to the step (c). If the entry is note detected by the step (f), the request from the second user terminal is rejected.